If it is a home cinema projector it will be native 480p, 720p or 1080p which are 16:9. As mjg100 said it will display the 4:3 image with the same height as the 16:9 image, but with black bars to the sides.
If it is a bussiness presentation SVGA or XGA projector then it is native 4:3, it will display a 16:9 image with the same width as the 4:3, but with black bars above and below. A XGA projector may have the option to display the 4:3 image as "real" rather than expanded to XGA, in that case it will display the 4:3 image with black bars to the sides and above and below, in the middle of the area it would have used for a 16:9 image.